1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, such as a copying machine, a facsimile machine, a plotter machine, or a multifunction peripheral including at least two of these machines, including the optical scanning device.
2. Description of the Related Art
Electrophotographic image forming apparatuses for use in a laser printing machine, a laser plotter machine, an ordinary-paper facsimile machine, and a multifunction peripheral providing at least two functions of these machines have become increasingly compact.
Under such circumstances, optical scanning devices having a wide angle of view and a short optical length from a photosensitive member have also been provided. For instance, optical scanning devices, in which a light ray (light beam) for use by a synchronization detector for synchronization detection does not pass through a scanning lens to achieve wider angle of view, have been proposed (see, for example, Japanese Patent No. 3768840 and Japanese Patent Application Laid-open No. H10-048554).
However, in the optical scanning device disclosed in Japanese Patent No. 3768840, an end portion of a scanning optical element is notched to allow passage of light rays for use in synchronization detection therethrough. Accordingly, when the angle of view for scanning increases, the synchronization light rays pass through a vicinity of the scanning optical element, which makes it difficult to separate scanning light rays from the synchronization light rays.
In the optical scanning device disclosed in Japanese Patent Application Laid-open No.H10-048554, a scanning optical element and an optical element for synchronization detection are independent from each other, which allows easy separation of scanning light rays from synchronization light rays. However, this technique requires an optical system for synchronization detection to be arranged in a small space between a light-supplying optical system and a scanning optical system, which are arranged between a light source and a deflector. Accordingly, the technique disclosed in Japanese Patent Application Laid-open No.H10-048554 is disadvantageous in that as the angle of view increases, configuring a layout of the optical systems becomes difficult.
With proliferation of high-speed image forming apparatuses capable of forming color images, tandem image forming apparatuses including a plurality of (typically four) photosensitive members have come into wide use. In such a tandem image forming apparatus, for instance, four photosensitive members are arranged along a transfer belt (or an intermediate transfer belt) that conveys a recording medium. After an electrostatic charging unit has electrostatically charged each of the photosensitive members, a latent image is formed on each of the photosensitive members by a writing unit. The latent images on the photosensitive members are individually developed with developing agents of different colors (for instance, with yellow, magenta, cyan, and black toners) into developed images. These developed images of different colors are transferred onto the recording medium (or the intermediate transfer belt) conveyed by the transfer belt such that the images are overlaid on one another, thereby forming a color image.
Examples of electrophotographic color image forming apparatuses include what is called as a one-drum-intermediate-transfer-type image forming apparatus that includes only a single photosensitive member. This type of image forming apparatus rotates the photosensitive member a number of colors turn to transfer images onto on an intermediate transfer member such that the images are overlaid on one another, thereby forming a color image on the intermediate transfer member, and thereafter transfers the images onto a recording medium. To form four-color images with such a one-drum image forming apparatus, it is necessary to rotate the photosensitive member four turns for each sheet of the color images. Accordingly, one-drum image forming apparatuses are inferior in productivity to tandem image forming apparatuses.
As discussed above, tandem image forming apparatuses are capable of high-speed image forming as compared with one-drum-intermediate-type transfer image forming apparatuses and therefore increasing productivity in color-image forming. However, a typical tandem image forming apparatus disadvantageously uses a plurality of light sources (for instance, generally four light sources are used in a tandem image forming apparatus that includes four photosensitive members) because optical writing on each of a plurality of photosensitive members is performed by a writing unit that uses optical scanning devices. This disadvantageously results in an increase in the number of components, misregistration due to different wavelengths of the plurality of light sources, an increase in production cost, and the like.
Deterioration of a semiconductor laser is one of typical causes of a failure of the writing unit. Accordingly, the larger the number of light sources, the probability of failure increases while recyclability decreases.
Under such a circumstance, a technique for not increasing the number of light sources of an optical scanning device for use in a tandem image forming apparatus is disclosed in, for example, Japanese Patent Application Laid-open No. 2002-023085. In this conventional technique, different to-be-scanned surfaces are scanned with light beams emitted from a single light source by using a pyramidal mirror or flat mirrors. However, this technique is still disadvantageous in that, although the number of light sources can be reduced, the number of deflecting mirror surfaces is limited to two at maximum, which can be a restriction on speedup.
An example of optical scanning devices configured to solve the problem discussed above is disclosed in Japanese Patent Application Laid-open No. 2006-284822. In this optical scanning device, a light ray emitted from a single light source is split by a beam splitter into two light rays that are away from each other in the sub-scanning direction. The light rays are deflected by a deflector including two polygon mirrors, which are stacked in an angularly-staggered arrangement and to be coaxially rotated, so as to scan two different to-be-scanned surfaces.
In this conventional technique, the two polygon mirrors stacked in the staggered arrangement in different phases are used as means that scans different to-be-scanned surfaces with light rays emitted from a single light source. However, this technique can increase production cost because such polygon mirrors stacked in the staggered arrangement in the different phases are not off-the-shelf items. Furthermore, this technique can cause degradation in image quality to occur because of involving processing of the polygon mirrors for causing an upper one and a lower one of the polygon mirrors to have different tilt surface angles and profile irregularities.
Another example of the optical scanning devices configured to solve the problem discussed above is disclosed in Japanese Patent Application Laid-open No. 2008-257169. In this optical scanning device, a light ray emitted from a single light source is split by a beam splitter into two light rays that are away from each other in the main-scanning direction. The two light rays are reflected from incident-light mirrors that cause the two light rays to impinge on a deflector having four reflecting surfaces in a manner that an angle between the two light rays incident on the deflector is 90 degrees. Hence, this configuration allows different to-be-scanned surfaces to be scanned with light beams emitted from the single light source. This conventional technique has solved the problem pertaining to the technique discussed above because a general-purpose polygon mirror having four reflecting surfaces can be used. However, in the configuration where the two light rays to impinge on the deflector with the 90-degree angle between the two incident light rays incident, an angle between each incident light beam and a center of a scan image relative to the deflector is restricted to approximately 45 degrees. When the angle of the incident beam is restricted to 45 degrees, an angle, between the center of a scanned image and an image end on the incident beam side, where image scanning can be performed is restricted to within 45 degrees. Put another way, the angle of view is undesirably narrowed. When, furthermore, synchronization detection for determining when to start image writing is performed at a position between the incident beam and the image end, the angle of view is further narrowed.
In recent years, widening the angle of view has been desired for compact design of writing units. A decrease in the angle of view is also a problem to be solved in the techniques discussed above.